Bortle Scale

 Bortle Scale

Astronomy is a field in which even an amateur can contribute. One such contribution is 'Bortle Scale' developed by John E. Bortle, an American amateur astronomer.

Bortle Scale is a numeric scale ranging from class-1 to class-9 that measures the darkness of night sky of a particular location. It relates and quantifies the observability of celestial bodies and the interference caused by light pollution.

The Bortle Scale has an inverse relation with the Naked Eye Limiting Magnitude (NELM) and direct relation with the light pollution of an area. It means when we go from class-1 (excellent dark sky site) to class-9 (inner city sky) of the Bortle Scale the light pollution increases and thus NELM decreases from 8.0 to 4.0. 

This scale was first published in the February 2001 edition of Sky and Telescope magazine. Bortle made this scale to help amateur astronomers in evaluating the darkness of observing sites and to compare their night sky darkness.

Below is the Bortle Scale :-

1. Excellent Dark-Sky Site

 Class- 1
 Title- Excellent Dark-Sky Site
 NELM- 7.6- 8.0
 Sky Description:-

• zodiacal light is colorful and visible
• the gegenschein is visible
• the zodiacal band is visible and spanning the entire sky
• airglow is readily visible
• the Scorpius and Sagittarius regions of the Milky Way cast obvious diffuse shadows on the ground
• many constellation particularly fainter ones, are barely recognizable due to large number of stars
• many Messier and globular clusters are naked- eye objects
• M-33 is a naked- eye object even with direct vision
• limiting magnitude with 12.5'' reflector is 17.5 with effort

2. Typical Dark Sky

 

Class- 2
Title- Typical Truely Dark Site
NELM- 7.1- 7.5
Sky Description:-

• the zodiacal light is distinctly yellowish and bright enough to cast weak shadows at dusk and dawn
• airglow may be weakly visible near horizon
• clouds are only visible as dark holes against the sky
• surroundings are barely visible silhouetted against the sky
• the summer Milky Way is highly structured
• many Messier objects and globular clusters are naked- eye objects
• M-33 is easily seen with naked- eye direct vision
• limiting magnitude with 12.5'' reflector is 16.5

3. Rural Sky

Class- 3
Title- Rural Sky
NELM- 6.6- 7.0
Sky Description:-

• the zodiacal light is striking and color is still visible (but weak) in spring and autumn
• some light pollution evident at the horizon
• clouds are illuminated near the horizon and dark overhead
• nearer surroundings are vaguely visible
• the summer Milky Way still appears complex
• M-15, M-4, and M-22 are naked- eye objects
• M-33 is easily visible with averted vision
• limiting magnitude with 12.5'' reflector is 16

4. Rural/ Suburban Transition

Class- 4
Title- Rural/ Suburban Transition
NELM- 6.1- 6.5
Sky Description:-

• the zodiacal light is still visible but does not  extend halfway to the zenith at dusk or dawn
• light pollution domes visible in several directions
• clouds are illuminated in the directions of the light source and are dark overhead
• surroundings are clearly visible even at a distance
• the Milky Way well above the horizon is still impressive but lacks detail
• M-33 is difficult to detect even with averted vision and is only visible when it is high in the sky
• limiting magnitude with 12.5'' reflector is 15.5

5. Suburban Sky

Class- 5
Title- Suburban Sky
NELM- 5.6- 6.0
Sky Description:-

• only hints of zodiacal light are seen on the best nights in autumn and spring
• light pollution is visible in most, if not all, directions
• clouds are noticeably brighter than the sky
• the Milky Way is very weak or invisible near the horizon and looks washed out overhead
• when it is half moon (first/last quarter) in a dark location the appears like this, but with the difference that the sky appears dark blue
• limiting magnitude with 12.5" reflector is 15

6. Bright Suburban Sky

Class- 6
Title- Bright Suburban Sky
NELM- 5.1- 5.5
Sky Description:-

• the zodiacal light is invisible
• light pollution makes the sky within 35° of the horizon glows grayish white
• clouds anywhere in the sky appears fairly bright
• even high clouds (cirrus) appear brighter than the sky background
• surroundings are easily visible near the zenith
• M-33 is not visible and M- 31 is modestly apparent
• limiting magnitude with 12.5" reflector is 14.5

7. Suburban/ Urban Transition

Class- 7
Title- Suburban/ Urban Transition
NELM- 4.6- 5.0
Sky Description:-

• sky looks light gray due to light pollution
• strong light sources are evident in all the directions
• clouds are brightly lit
• the Milky Way is invisible
• M-31 and M-44 may be glimpsed but with no detail
• if one looks through a telescope, the brightest Messier objects are pale ghosts of their true selves
• when it is full moon in a dark location the sky appears like this, but with the difference that the sky appears blue
• limiting magnitude with 12.5" reflector is 14

8. Sky City

Class- 8
Title- Sky City
NELM- 4.1- 4.5
Sky Description:-

• the sky is light grey or orange in color (one can easily read a text written on a paper during night)
• stars forming familiar constellation patterns mat be weak or invisible
• M-31 and M-44 can be barely glimpsed by an experienced observer on good observing nights
• only bright Messier objects can be detected with a telescope
• limiting magnitude with 12.5" reflector is 13

9. Inner-Sky City

Class- 9
Title- Inner-Sky City
NELM- 4
Sky Description:-

• the sky is brilliantly lit
• many stars forming constellations are invisible and many fainter constellations are invisible
• aside from the Pleiades, no Messier object is visible to the naked eye
• the only objects to observe are the Moon, the planets and a few of the brightest star clusters